PREHEAT the oven to 350°F.
MAKE THE CARNITAS: In a Dutch oven or large ovenproof pot, combine the orange juice, lime juice,cumin, paprika, coriander, garlic, and salt. Add the pork and stir to combine. Bake,covered, until very tender, about 2½ hours.
USING a slotted spoon, transfer the pork to a cutting board. Use two forks to shred the pork into small pieces and return it to the pot. Simmer uncovered, over medium-low heat, until almost all of the liquid is gone, about 20 minutes.
MAKE THE TORTILLAS: Meanwhile, thinly slice the celery roots with a mandoline on the ⅛-inch setting or with a sharp knife. Choose the 18 largest slices for tortillas. (Store any leftover slices in an airtight container in the refrigerator for another use.)
HEAT a grill pan over medium heat. Cook, turning occasionally, until grill marks form and the tortillas are tender and pliable, about 5 minutes. (You may need to press gently on the tortillas with a spatula.)
TO serve, spoon about 1 tablespoon of the meat into each tortilla. Top with guacamole, salsa, and Sriracha, if desired.Tip: This recipe makes about 4 cups of carnitas, so you will have leftovers. Store the meat in an airtight container in the refrigerator for up to 3 days.